In the oil and gas industry it is necessary to be able to record different states in the process in order to suitably control the process. In connection to the separation of water, gas and sand from oil in gravity separators, the sand will normally accumulate as one or more heaps or a layer or bed at the bottom of the separator tank as a result of gravitational settling.
In order to control the conditions in the separator tank by controlling the amount of sand gathered at the bottom thereof some kind of measurement or estimation of the amount as well as the distribution of sand is required.
U.S. Pat. No. 4,912,977 discloses a device for position determination of transition zones between at least two different enclosing media. More specifically it discloses a measuring device arranged for determination of the vertical distribution of one or more media which have a thermal connection with the device and where the positions of the interfaces can be determined on the basis of differences in thermal states and properties, comprising an arrangement of one or more thermal conductor elements, which have a common thermal reference element and discrete, chiefly thermally insulated sensor areas which have good thermal contact with the medium, being measured through a common partition wall which affords sufficient physical and thermal separation, as well as means for keeping the reference element at a temperature level different from the medium being measured. The temperature sensing elements may be composed of a thermopile, the thermal stress of which generates a voltage that is transmitted to a signal processor for further analysis.
However, this prior art device, though useful for determination of transition zones between for example oil, water, foam and gas in a gravity separator tank, is deemed to be rather complicated for the purpose of measuring the presence of a sand layer or sand heap at the bottom of such a tank. Also, for the purpose of covering a large area of the bottom of the tank, a plurality of such devices would be needed, further increasing the cost.